1. Field of the Invention
The present invention relates to a semiconductor device where a circuit comprising thin film transistors is formed over a substrate having an insulating surface, and a manufacturing method thereof.
2. Description of the Related Art
Display devices of which pixel portion is constructed by arranging active elements are referred to as active matrix display devices. Among them, a liquid crystal display device and an electroluminescence (hereinafter also referred to as EL) display device and the like have been developed. As an active element, a gate insulated transistor is typically used, and preferably a thin film transistor (hereinafter also referred to as a TFT) is used. A TFT is obtained by forming a semiconductor film over a substrate such as glass by vapor phase growth and the like, and then forming a channel region, a source region, a drain region and the like using the semiconductor film. The semiconductor film is preferably formed of silicon or a material such as silicon germanium that contains silicon as its main component. The semiconductor film can be classified into an amorphous semiconductor film and a crystalline semiconductor film.
In a TFT whose active layer is formed of an amorphous semiconductor film, it has been almost impossible to obtain field effect mobility of 10 cm2/V·sec or more because of the electron properties of the amorphous structure. Therefore, even though the TFT can be used as a switching element (hereinafter also referred to as a pixel TFT) for driving liquid crystals in a pixel portion of an active matrix liquid crystal display device, it is impossible to construct a driver circuit for image display by using such a TFT. Thus, mounting techniques of driver ICs using TAB (Tape Automated Bonding) or COG (Chip on Glass) bonding are adopted for the driver circuit.
As for a TFT whose active layer is formed of a crystalline semiconductor film, on the other hand, high field effect mobility is obtained, which can thus be used for constructing various functional circuits to be driven. Thus, a driver circuit comprising a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit and the like can be formed over the same glass substrate as the pixel TFTs. A driver circuit is basically constructed by a CMOS circuit comprising an N-channel TFT and a P-channel TFT. Based on such mounting techniques of a driver circuit, it is considered that a TFT whose active layer is formed of a crystalline semiconductor film, which enables the integral formation of a pixel portion and a driver circuit over the same substrate, is suitably employed in order to advance weight saving and slimming of a liquid crystal display device or an EL display device.
As set forth above, in order to form a pixel portion and a driver circuit over the same substrate, TFTs corresponding to the function of each of the various circuits are required to be formed. This is because, operating conditions of pixel TFTs are not necessarily identical to those of TFTs in a driver circuit, and the respective TFTs are thus required to have different properties. Pixel TFTs constituted by N-channel TFTs are used as switching elements to apply a voltage to liquid crystals. The liquid crystals are driven by a DC voltage, therefore, a method called a frame inversion drive is often adopted. A pixel TFT is required to have a sufficiently small off-current in order to store a charge accumulated in a liquid crystal layer within one frame period. On the other hand, a buffer circuit and the like in a driver circuit is applied with a high drive voltage, therefore, a withstand voltage is required to be increased so that the elements are not broken by the high voltage applied. In addition, in order to obtain a high on-current drive capacity, it is necessary to secure a sufficiently large on-current.
As a structure of a TFT for decreasing off-current, there is the one having a low concentration drain region (hereinafter also referred to as a LDD region). This structure has a region doped with impurity elements at a low concentration between a channel region and each of source and drain regions that are doped with impurity elements at high concentration. Also, there is a so-called GOLD (Gate-drain Overlapped LDD) structure in which a LDD region is formed to overlap a gate electrode with a gate insulating film interposed therebetween as a means for preventing on-current from decreasing due to hot carriers. According to such a structure, a high electric field in the vicinity of a drain is alleviated, thereby it becomes possible to prevent on-current from decreasing due to hot carriers. Note that the LDD region of the portion that does not overlap the gate electrode with the gate insulating film interposed therebetween is referred to as a Loff region while the LDD region of the portion that overlaps the gate electrode with the gate insulating film interposed therebetween is referred to as a Lov region.
Here, the Loff region works effectively in suppressing off-current whereas it does not work effectively in preventing on-current from decreasing due to hot carriers by alleviating the electric field in the vicinity of the drain. On the other hand, the Lov region works effectively in preventing on-current from decreasing by alleviating the electric field in the vicinity of the drain, however, it does not work effectively in suppressing off-current. Accordingly, TFTs having appropriate TFT characteristics for each of the various circuits are required to be formed.
However, manufacture of TFTs corresponding to the function of each of the various circuits makes structures thereof complex, which involves a larger number of manufacturing steps. Such an increase in the number of the manufacturing steps leads to a higher manufacturing cost and a lower manufacturing yield.